Has anybody build their own corn burning furnace or has some of the
details,
like size of fire pot, air holes size and placement. I am interested in
an under fed system. Any other details that I have not listed would be
appreciated.

A number of folks in my area have copied the design of corn stoves for sale.
I seen a really nice one at the MN state fair last summer. Three problems
have cropped up with the stoves I'm aware of:

1. Heat cracks and bends the agitator fingers. This could be solved by using
only 304 SS for this component.

2. Ash buildup is a serious problem. Daily cleaning is required with some
lots of corn. Combustion efficiency goes to heck when the air holes are
plugged. Oyster shells have helped, but not enough.

3. Most units only produce 50,000 BTU on a good day, too small a heat
source. You'll still need a backup.

I was talking this over with a friend last fall. Someone should look into
building a unit that works like a coal boiler: Grind the corn up, feed into
a firebox with a strong air flow from below. This is called a fluidized bed.
The ash goes out the stack. Making a working design small enough would be
the biggest challenge here, most commercial coal unit are for 1,000,000s of
BTUs

Karl